The present invention relates to an electron microscope device, by which it is possible to observe a scanning electron image and an optical image of a specimen. In particular, the present invention relates to an electron microscope device, by which an optical image can be observed at the same time during electron scanning action.
A scanning electron microscope (SEM) is so designed that an electron beam is projected for scanning over a specimen, and the electron issued by the projection of the electron beam is detected. Then, a scanning electron image is acquired on detailed status of a surface of the specimen.
On the other hand, a field angle of the electron beam is very small, and it is very difficult to project the electron beam to a specific position unless a projecting position is identified and recognized in advance.
For this reason, an electron microscope device is provided with an optical microscope, which has magnification lower than magnification of the electron microscope. First, an illumination light (white light) is projected to the specimen, and the specimen is observed by means of the optical microscope, and a projected position is specified. Then, by changing over to the electron microscope, the electron beam is projected to scan over the projected position of the specimen and the specimen is observed.
In case it is wanted to obtain a scanning electron image, electrons emitted from the specimen enter to a fluorescent substance. A light emitted from the fluorescent substance is converted to an electric signal by a photoelectric conversion element, and a scanning electron image is acquired based on this electric signal. In case an optical image is acquired by using an optical microscope, a light reflected from the specimen is received by a photoelectric element for the optical microscope and the detection result is converted to an electric signal, and an optical image is acquired based on this electric signal.
However, a level of energy is extremely different between electrons acquired by scanning of the electron beam and a light reflected from the specimen. When a reflection light from the specimen enters the photoelectric element, which is detecting the electron beam, the photoelectric element is saturated or S/N is extremely low, and the electron beam cannot be detected.
Therefore, it has been practiced in the past that the optical microscope and the electron microscope are changed properly between a case where the optical microscope is used for observation and a case where the electron microscope is used, and it is so arranged that the electron microscope is not used for observation when it is observed by the optical microscope and that the optical microscope is not used for observation of the specimen when it is observed by electron microscope.
For instance, the optical microscope and the electron microscope are separated from each other, and an optical axis of the optical microscope and an optical axis of the electron microscope are set in a known specific relationship. When a table on which the specimen is placed is moved between the optical microscope and the electron microscope, a position to observe an optical image is associated with a position to observe a scanning electron image.
Therefore, there arises a problem in a conventional type electron microscope that the structure is very complicated and the scanning electron image and the optical image cannot be observed at the same time.
An electron microscope, by which the scanning electron image and the optical image can be observed at the same time, is disclosed in JP-A-4-280053.
In the electron microscope described in JP-A-4-280053, an optical system of the electron microscope and an optical system of the optical microscope have the same optical axis. An illumination light and an electron beam are projected at the same time. In the signals from the electron detector, signals of the illumination light are removed as DC components, and signals containing only electrons are extracted.
However, the DC components of the illumination light are extremely in higher amount compared with signal components, and it is very difficult to attain the purpose.